The present invention relates to the use of a high strength stainless construction steel for making of components for the process industry, especially components intended to be exposed to caustic soda. As these materials are applicable to various forms of equipment used in applications where exposure to caustic soda is found, they are hereinafter called "construction material" which should be understood to mean tube, sheet, bar or any other form the material may take in that environment.
Ferritic-austenitic (duplex) stainless steels are today widely used as construction material in a number of industrial applications. The duplex steel, which is described in U.S. Pat. No. 5,582,656 (the content of which is hereby incorporated by reference) and characterized in having a composition comprising 28-35% Cr, 3-10% Ni, 1.0-4.0 Mo and 0.2-0.6% N, has been found to have especially favorable and in some cases surprisingly good properties as a construction material in specific applications. The present invention relates to the use of the steel grade as a construction material within areas where sodium hydroxide (NaOH) and solutions thereof are used or prepared, and where especially favorable properties have been achieved. The alloy according to U.S. Pat. No. 5,582,656 is identified by the trademark SAF 2906 and preferably contains 29-33% Cr, 3-7% Ni and 1-3% Mo.
High corrosion rates can arise when steel and also other metal such as titanium, are exposed to NaOH. In solutions with 30-48% NaOH, and at temperatures between 100 and 200.degree. C., the corrosion rate of chromium steels diminishes with increasing chromium content of 24% and above. The corrosion rate of chromium steel can also be reduced by the addition of sodium chlorate (NaClO.sub.3) which has been proved for instance for 48% NaOH and a chromium content of at least 5% in the chromium steel, and an addition of 0-1% NaClO.sub.3.
When using austenitic stainless steels in solutions containing NaOH, there exists, apart from high corrosion rates, also a great risk of stress corrosion cracking, which risk diminishes with an increasing amount of nickel in the material.
Caustic soda is produced, among other methods, by the electrolysis of sodium chloride solutions, whereby also chlorine gas is produced. When the diaphragm method is used, the achieved sodium hydroxide has a concentration of about 12% after the electrolysis and it contains also chlorides and in some cases chlorate. The concentration of the solution is increased by evaporation in several steps in so called NaOH evaporators. The solution is heated in these evaporaators under pressure, steam evaporates and chlorides are removed (after crystallization). The concentration of NaOH reaches up to 47% and higher in the last evaporation steps. In spite of the fact that chlorides have been removed in several steps remaining amounts of both solid (up to 8%) and dissolved chloride (up to 7%) have been shown The temperature in the NaOH evaporators with the highest NaOH concentration is 160-170.degree. C.
The corrosion rates in a NaOH evaporator can vary greatly, above all depending-on temperature and flow rates. The temperature, variations in the same part of an apparatus, for instance, an evaporator tube or a wear plate, can cause so high corrosion rates locally that an unacceptable low length of life is achieved while at the same time as the major part of the material only shows insignificant corrosion. At the normally occurring temperature differences, inside for instance a NaOH evaporator tube, or at temperature alteration in connection with process changes, materials which have previously shown good length of life can therefore show high corrosion rates, which result in a substantially reduced length of life for the material.
Especially high flow rates exist, apart from inside the evaporator tubes, also at the propeller of the circulation pump and at the air diffuser. These parts therefore runs the risk of being exposed to erosion corrosion, which is further aggravated with increasing amounts of undissolved salt in the solution, and consequently show high corrosion rates. Several materials, which have been used in the construction of NaOH evaporators, have in many cases, an unacceptable low length of life in these exposed parts.
The materials that now dominate in construction of NaOH evaporators are 26-1 (UNS S44626), Nickel 200 (UNS N02200) and Sanicro 28 (UNS N08028). Certain problems with high corrosion rates and erosion corrosion exist however, which lead to shortened life of length of the construction. Nickel 200, which essentially contains 0.15 C (max), 99 Ni, 0.10 Cu (max) and 0.20 Mg (max), is today the dominating choice of material thanks to its high resistance to corrosion by the process solution in NaOH evaporation. Tests with Sanicro 28 have been done, but the material has a limited length of life due to corrosion.
Nickel 200 has however the limitation that its erosion resistance is reduced compared to stainless steels as Sanicro 28. The combination of high erosion resistance and at the same time high resistance to the process solution in NaOH evaporation would therefore be optimal.